It's hot outside, so like a lot of people, you try to park your car under a tree in parking lots and on the street for some shade. You choose to park under a big hackberry tree, but when you return to your car, you notice droplets on your windshield and sticky stuff on the sidewalk, other cars, and the parking lot. What is this?

The sticky substance is called honeydew. The honeydew is excreted by a number of sap-sucking insects such as aphids, whiteflies, mealybugs, certain scale insects, and few others. On hackberry trees (widely planted in some cities), an insect called the woolly hackberry aphid produces a large amount of honeydew.

If you examine the tree's leaves, you may see bluish-white masses that are actually...